I don’t always agree with dirty hairs being the best option for coloring hair but a dirty scalp is significantly important. The hairs and the scalp absorbs color differently making a colorist keenly aware of reactions and results.
Yes, an oily scalps is a natural means of safety for our scalps against dangerous chemicals. The layer of oil that some attempt to quickly rid of is good. Our sebaceous glands beneath the scalp are working to protect, protect and yes, protect. Since permanent hair color and hair lightners are significantly higher than the pH (potential hydrogen) of skin, a dirty scalp helps to be a protective barrier. Natural oils are not an insurance policy from burns or irritations yet better than bare skin. Hair color is to be taken seriously.
A patch test is advise.
I question the theory of just dirty hairs being the best hair to color. There are many issues that would stop hair color from penetrating or yielding consistent results such as heavy styling products (products that are not water based) coating the hair.
In such cases the root hairs will be a true color and the hair shaft to ends will yield something else. A prime example is hot roots as some describe it. A buildup on the ends and hair shaft yields an unpredictable color. A reason to bust the myth of dirty hairs or buildup being the suitable option.
Hair color (pigments) and developers ( ammonia and hydrogen peroxide) are manufactured to be alkaline to open the outermost layer, the cuticle. The alkaline level of hair color ranges from 7 to 11 on the pH scale.
An important note: the pH scale ranges from 0 to 14. High alkaline levels raising the pH on our skin which is normally resting between 4.5 to 6.5 and sometimes 7 is a reason for concern.
So I challenge the theory, our scalps are taking just as much a beating as our hairs. Hairs on our heads are for adornment and protection. A better question to have a scientific explanation of is what are the combination of past, present and future colors doing to our scalps long term?